Special Weather Statement, NWS Phoenix, AZ

Area Forecast Discussion
National Weather Service Phoenix AZ
211 PM MST Tue Jul 20 2021

.SYNOPSIS...
Abundant monsoon moisture will continue to bring daily chances for
at least isolated to scattered showers and thunderstorms across much
of Arizona with a tendency to mostly favor the higher terrain of
eastern Arizona, especially for the first half of the week. Near
normal temperatures are favored through midweek, followed by
below normal by late in the week. More scattered to possibly
widespread storms are expected for the second half of the week and
into the weekend as a more organized weather system is expected to
affect the region. Storms will be capable of producing both
locally heavy rain, strong winds, and blowing dust.

&&

.DISCUSSION...
Showers and thunderstorms scooted across the Phoenix metro and
southwest AZ/southeast CA this morning, dropping 0.10 to 0.25" of rain
over a few fortunate spots, in response to a passing inverted trough
and nearly 1500 j/kg of CAPE. Often, morning convection will spoil
storm chances for the afternoon and the morning HREF showed a relatively
quiet day for our forecast area. However, latest radar trends indicate
that may not be the case with multiple thunderstorms now developing
southwest, east, and north of the Phoenix metro. Storms forming
over the higher terrain is not surprising but does cause some pause
as to whether or not we can get more storms into the lower desert
today than previously thought. The latest aircraft soundings show
less inhibition with a weaker midlevel cap and much more instability
than guidance suggested...but those soundings may not be adequately
sampling the storm environment either. Lower desert storm chances
for this afternoon are ~10-25%.

Overall, water vapor analysis shows a ridge of high pressure centered
over the Colorado Rockies with a fairly moist air mass stretching
from tropical Mexico through the Intermountain West. The H5 ridge
center will continue to weaken while settling close to the Four
Corners region over the next day or two, providing stronger
southeasterly flow through southern Arizona. This position will keep
the monsoon in gear with chances for thunderstorms seemingly every
day. PV anomalies associated with a modest midlevel jet streak
over western TX and New Mexico are expected to move through the
region later this week, potentially increasing chances for
thunderstorms perhaps as early as Wednesday evening, but more
likely Thursday and/or Friday.

Aside from the aforementioned 1500 j/kg CAPE, the 12z KPSR sounding
also showed a whopping 2.13" in precipitable water, exceeding the
daily maximum for date according to SPC Sounding Climatology. Daytime
mixing should lower PWs somewhat, but not by all that much, as the
latest aircraft soundings out of Sky Harbor also measured 1.98"
PW. These moisture levels are much more typical of a late August
tropical airmass rather than July 20th. This ultimately means that
any thunderstorms that do develop will have the potential for
producing heavy rain that could cause flash flooding. To illustrate,
a thunderstorm that developed rapidly over the Bradshaw Mountains
around noon dumped nearly 3.00" of rain over one gauge within one
hour, and exceeded rainfall rates of 1.00"/hr over a few other rain
gauges! That does not mean every storm will dump that much rain but
it does not take nearly that much rain to cause flooding in our arid
environment. The threat for flash flooding is anticipated to increase
daily through the end of this week as storm chances and coverage are
also forecast to increase.

Temperatures through Wednesday are expected to be near seasonal
norms to slightly above normal across western deserts before
dropping several degrees as the storm chances increase and the
upper low moves into the area late week. Latest NBM has Wednesday
as the warmest day and Friday and Saturday as the coolest. NBM is
even supporting a 30% chance for sub-90 degree highs Friday and
Saturday in Phoenix. Temperatures should warm back up during early
next week as the pattern continues to shift with ensemble
guidance supporting a transition to increased south to southwest
flow and a gradual drying trend.

&&

.AVIATION...Updated at 1749Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT: Appears
to be a rather quiet day aviation-wise across the Phx area. Current
light/variable winds to become solidly westerly by around noon with
these westerly winds continuing into the evening hours. A more
diurnal shift to easterly late this evening/shortly after midnight
is now anticipated as latest high-res model output is now keeping
convective activity/outflows well outside of the Phx area. As far as
Wednesday is concerned, expecting easterly winds to persist into the
afternoon hours with a typical diurnal switch to westerly during the
mid/late afternoon hours. Prospects for convective activity
impacting the terminals look fairly good for Wed evening (30-40%)
with an even better chance for easterly outflows (60-70%) with a 1st-
guess timing of a wind switch to easterly being during the 02-04Z
time period.

Southeast California/Southwest Arizona including KIPL and KBLH:
Current shower activity in the vicinity of both airfields is
expected to diminish/move off to the west/north by 19Z. Winds will
generally retain a southerly component at both KIPL and KBLH through
Wednesday morning. Otherwise, convection is generally not expected
this afternoon, except for the possibility of an isolated shower or
thunderstorm near KBLH this afternoon or evening. A brief gust to 30
kt along with a brief reduction in cloud bases to 8k ft is possible
in any storms that develop.

&&

.FIRE WEATHER...
Friday through Tuesday:
No major changes to the forecast...Lower than normal afternoon
temperatures and ample monsoonal moisture will persist. Storm
activity is anticipated to spread as far west as southeast
California with most storms having good potential for wetting rain
(0.10" or more). The best potential for more widespread storm
activity will likely be Thursday through Friday and into Saturday
as an upper level disturbance moves through the region. Threats
from storms will continue to involve locally heavy rainfall and
flash flooding along with strong outflow winds and localized
downbursts within storms. Significant flooding may be experienced
on previously burned areas of Gila County, particularly during the
second half of the week. Min RH levels are expected to be in the
25-40% range across south-central Arizona (30-50% higher terrain
of south- central AZ) and 20-30% across southeast California and
southwest Arizona. Max RH levels, outside of storms, will be
45-70% most places (60-90% higher terrain of south-central AZ).
Apart from thunderstorms, winds will favor familiar diurnal
patterns.

&&

.SPOTTER INFORMATION STATEMENT...
Spotters should follow standard reporting procedures.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...None.
CA...None.
&&

DISCUSSION...AD/Benedict
AVIATION...Percha/Hirsch
FIRE WEATHER...Smith/Kuhlman

Special Weather Statement
National Weather Service Phoenix AZ
141 PM MST Tue Jul 20 2021

AZZ547-549-552-555-557-558-202130-
Fountain Hills/East Mesa AZ-Mazatzal Mountains AZ-
Apache Junction/Gold Canyon AZ-Superior AZ-Rio Verde/Salt River AZ-
Pinal/Superstition Mountains AZ-
141 PM MST Tue Jul 20 2021

...SIGNIFICANT WEATHER ADVISORY FOR MARICOPA AND PINAL COUNTIES UNTIL
230 PM MST...

At 141 PM MST, Doppler radar was tracking a strong thunderstorm near
Canyon Lake, or 10 miles northeast of Gold Canyon, moving northwest
at 10 mph.

Pea size hail and wind gusts of 50 to 55 mph will be possible with
this storm.

Locations impacted include...
Apache Junction, Saguaro Lake, Canyon Lake, Superstition Mountains,
Lost Dutchman State Park, Goldfield, Apache Lake, Gold Camp and
Tortilla Flat.

LAT...LON 3342 11106 3331 11114 3345 11159 3358 11156
      3371 11141
TIME...MOT...LOC 2041Z 120DEG 7KT 3350 11136

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Special Weather Statement
National Weather Service Phoenix AZ
257 PM MST Tue Jul 20 2021

AZZ539-559-202245-
Sonoran Desert Natl Monument AZ-Gila Bend AZ-
257 PM MST Tue Jul 20 2021

...SIGNIFICANT WEATHER ADVISORY FOR MARICOPA COUNTY UNTIL 345 PM
MST...

At 256 PM MST, Doppler radar was tracking a strong thunderstorm 11
miles southeast of Sentinel, or 25 miles southwest of Gila Bend. This
storm was moving northwest at 10 mph.

Pea size hail and wind gusts of 50 to 55 mph will be possible with
this storm.   Blowing dust and reduced visibilities down to 2-4
miles will also be possible.

This storm will remain over mainly rural areas of Maricopa County.
This includes AZ Interstate 8 between mile markers 81 and 98.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Torrential rainfall is also occurring with this storm, and may lead
to localized flooding. Do not drive your vehicle through flooded
roadways.

&&

LAT...LON 3266 11296 3258 11305 3285 11333 3292 11302
TIME...MOT...LOC 2156Z 136DEG 3KT 3277 11307

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